The present invention relates to a phase-change optical recording element that is particularly suitable for bum-dark type write-once read-many times (WORM) applications.
Optical recording has been increasingly used in recent years to publish, distribute, store, and retrieve digital information. This is done by focusing a laser beam to write and/or read information on an optical recording element usually in the form of a spinning disk. In the read-only memory (ROM) format, the information is prefabricated at the factory in the form of encoded small features on the element and the laser beam is used to read back the information. In the writeable formats, the laser beam is used to create the small encoded marks through a variety of physical recording mechanisms. This permits a user to record their own data on the disk. Some recording physical mechanisms are reversible. The recorded marks can be erased and remade repeatedly. Disks that utilize these mechanisms are called Erasable or Re-writeable disks. Some of these physical mechanisms are one way, once the marks are made they cannot be reversed or altered without leaving a clearly identifiable trace that can be detected. Disks that use these mechanisms are called WORM (Write-Once-Read-Many times) disks. Each of these formats is suitable for certain practical applications.
The popularity of compact disk recordable (CD-R), a WORM disk, is because WORM disks are suitable for many applications. In some of these applications, the data need to be stored in such a form that any modification to the content is not possible without leaving an easily detectable trace. For example, attempts to record over a previously recorded area may result in an increase in the read-back data jitter. An increase in data jitter of 50% is easily detectable and can be used to identify a recording element that has been modified. Recording elements that possess features that allow detection of modification attempts are hereto referred to as true-WORMs. In some other applications such publishing and data distribution, the lower cost of WORM recording element makes them desirable. Yet in some other applications some performance advantages of WORM recording elements such as a higher writing speed becoming the determining feature in choosing WORM elements over rewriteable elements. Because of the success of the CD formats, currently CD-R (compact disc-recordable) is the most demanded WORM recording format. With the advent of other formats, WORM recording elements compatible with those formats will be in demand as well. For example, WORM disks based on the digital versatile disc (DVD) format, hereto referred to as DVD-WORM, are expected to be in demand because of the increasing popularity of the DVD format.
A useful DVD-WORM disk needs to be compatible with the DVD-ROM or DVD-Video disks. It needs to meet the many physical properties already defined for the DVD-format disks. In addition, for the disk to be widely accepted, the cost needs to be low. There is a need for an improved WORM optical recording element that will function successfully as a DVD-WORM.
Many physical mechanisms have been used for WORM recording. The first practical WORM optical recording element utilized ablative recording where the pulsed laser beam is used to create physical pits in the recording layer. This mechanism requires the recording elements to be in an air-sandwiched structure to leave the surface of the recording layer free from any physical obstruction during the pit formation process. This requirement not only increases the cost but also introduces many undesirable properties that severely limit the usefulness of the recording element. Another mechanism is to use the laser beam to cause the fusing or chemical interaction of several layers into a different layer. This mechanism suffers from the requirement of relatively high laser power.
Yet another approach is to use organic dye as the recording layer. Although used successfully in CD-R disks, this mechanism suffers from its strong wavelength dependence. The optical head used in the DVD devices operating at 650 nm, for example, is not able to read the CD-R disks designed to work at the CD wavelength of 780 nm. Furthermore, a dye-based recording element tends to require more laser power for recording, and may have difficulties supporting recording at high speeds.
A desirable approach is based on amorphous-crystalline phase-change mechanism. This phase-change mechanism is the basis for the re-writeable DVD disks that have been introduced as DVD-RAM and DVD-RW products in the market. A phase-change based DVD-WORM disk will have the best similarity in characteristics with the re-writeable DVD disks, and it can share the same manufacturing equipment with the re-writeable disks. Both of these are highly desirable. Since the WORM feature requires disks that cannot be re-written, different phase-change materials or disk constructions need to be used from those conventionally used for rewriteable disks. Commonly-assigned U.S. Pat. Nos. 4,904,577; 4,798,785; 4,812,386; 4,865,955; 4,960,680; 5,271,978, 4,774,170; 4,795,695; and 5,077,181 teach various alloys that can be used for write-once phase-change recording. When these alloys are used to construct a WORM optical recording element, the recording laser beam is used to change the atomic structure of the recording phase-change material from the amorphous state to the crystalline state. Optical recording elements based on these alloys have many advantages over other WORM optical recording elements. In particular, it can be used in a simple, single-layer construction that drastically reduces manufacturing costs. However, since the reflectivity of the crystalline state is usually higher that of the amorphous state, the recorded data marks appear brighter than the surrounding area. These optical recording elements are hereon referred to as xe2x80x9cburn-brightxe2x80x9d type optical recording elements. This is in contrast to ROM disks, such as DVD-ROM and DVD-video disks, which all have recorded marks with lower effective reflectivity than the surrounding area. These types of optical recording elements are hereon referred to as xe2x80x9cburn-darkxe2x80x9d optical recording elements. Drives that are designed to work with burn-dark type optical recording elements frequently cannot read burn-bright type optical recording elements. Thus, the burn-bright type phase-change optical recording elements are incompatible with the popular drives designed for burn-dark type ROM optical recording elements. This incompatibility limits the usefulness of the burn-bright type phase-change optical recording elements and is highly undesirable. This type of optical recording elements can therefore not be used as DVD-WORM.
It is a well-known requirement that DVD disks should have a reflectivity higher than about 18% and a contrast value higher than 0.60. Here, the contrast I14M is defined as
I14M=(I14Hxe2x88x92I14L)/I14H
wherein I14H is the reflectivity signal off the un-recorded amorphous region, and I114L is the reflectivity signal off a long recorded crystalline mark. All reflectivity signals were measured using the read laser beam in a fully recorded disk. It is believed that a reflectivity value of larger than about 28% as measured by a collimated beam on an unrecorded disk is needed to achieve the 18% reflectivity requirement. In DVD, the marks are sub-resolved meaning that the read laser spot is actually larger than the recorded marks. The I14L signal, therefore, is not just a measure of the crystalline state reflectivity, but also includes contribution from the surrounding amorphous area. Similarly, the I14H signal is a not just a measure of reflectivity from the un-recorded amorphous region but also it includes some contribution from recorded crystalline marks on the neighboring tracks. It is believed that with the reflectivity  greater than 18% requirement, the contrast of 0.6 is only achievable if the collimated-light reflectivity of the crystalline state is less than about 15%. Since the collimated-light data also includes reflected light from the front surface of the transparent substrate, the required reflectivity off the surface of the phase-change surface is actually less than 10%. A highly tuned optical structure such as a quadri-layer structure is therefore necessary to meet the requirements.
Nagata et al. (K. Nagata, S. Furukawa, Y. Kamioka, N. Yamada, and N. Akahira xe2x80x9cphase-shift recording characteristics of phase-change optical discxe2x80x9d, The 44th Spring Meeting; The Japan Society of Applied Physics and Related Societies, Mar. 31, 1997) teach the construction of a WORM disk using a phase-change material in a quadri-layer structure. The disk structure comprises a substrate onto which a dielectric layer, an amorphous state-change layer, another dielectric layer, and a metallic reflector layer are formed. The thickness and the materials of the layers are selected such that when the amorphous material crystallizes by the recording laser beam there is little change in reflectivity. Instead, a substantial change in the optical phase of the reflected beam is produced. Light reflected from the crystalline marks interferes destructively with that reflected from the surrounding amorphous area and the recorded marks appear dark upon read back. Although this approach is successful in producing dark recorded marks, the reflectivity of the un-recorded disk at around 10% is lower than the 28% value required to be compatible with the reflectivity of the DVD format disks.
It is an object of the present invention to provide WORM optical recording elements based on phase-change materials.
It is a further object of the invention to provide WORM optical recording elements that are the burn-dark type.
It is a further object of the invention to provide phase-change type WORM optical recording elements that do not need to be pre-crystallized before data can be recorded on the optical recording elements.
It is another object of the invention to provide phase-change type WORM optical recording elements that possess true-WORM characteristics in that once the data are written, they cannot be altered without being detected.
It is still another object of the invention to provide WORM optical recording elements that meet the contrast and other physical requirements of the DVD format disks.
These objects are achieved in a WORM optical recording element comprising:
(a) a substrate;
(b) an amorphous phase-change recording layer disposed over the substrate;
(c) a dielectric layer disposed adjacent to the amorphous phase-change layer;
(d) a reflector layer disposed adjacent to the dielectric layer; and
(e) wherein the material and the thickness of the layers are selected such that;
(i) recording can be performed on the optical recording element by using a focused laser beam to form crystalline marks in the phase-change layer using laser pulses of less than 40 nS in duration;
(ii) the reflectivity of the amorphous phase as measured by a collimated beam is higher than 28% and the contrast of the read-back signal is higher than 0.6; and
(iii) the second and subsequent writing over previous recording results in at least a 50% increase in read out jitter.
The present invention has advantages in that it can be used in WORM optical recording elements that do not require initialization (pre-crystallization) as part of the manufacturing process; it possesses true-WORM characteristics, and it can meet the established DVD physical requirements. It is to be understood that, although designed to function as a DVD-WORM optical recording element, the invention can be used for other WORM elements, such as an optical tape.